Neuro: The Chemical Senses Flashcards
List some examples of chemical senses.
- taste
- smell
- CO2/O2 levels - chemoreceptors in arteries of the neck
- chemical irritants - nerve endings in skin/mucous membranes
- acidity - sensory nerve endings in muscle e.g. lactic acid
Is taste innate or learned?
- Humans innately enjoy sweet flavours and avoid bitter flavours - this is evolutionarily ancient (e.g. distinguish food sources, avoidance of toxins)
- However, experience can strongly modify our innate preferences - humans can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)
What are the five basic tastes?
- sweet
- salty
- sour
- bitter
- umami
List the organs of taste (include sensory signals from smell/texture as well).
Palate - roof of mouth separating oral and nasal cavities. Taste buds present.
Epiglottis - leaf shaped cartilage covering laryngeal inlet (prevents food entering the windpipe and lungs when swallowing). Taste buds present.
Pharynx and Nasal Cavity - odours from food can pass via the pharynx to the nasal cavity to be detected by the olfactory receptors
Describe the tongue and its relation to taste.
- Tip of the tongue is most sensitive to sweetness, the back is most sensitive to bitterness and the sides are most sensitive to saltiness and sourness. However most of the tongue is sensitive to all 5 basic tastes.
- The surface of the tongue contains small projections termed papillae:
I. Ridge-shaped (foliage)
II. Pimple-shaped (vallate)
III. Mushroom-shaped (fungiform) - The papillae contain taste buds. Taste buds contains taste receptor cells There is also a taste pore. This is the chemically sensitive end of a taste receptor cell in which chemicals dissolved in saliva can interact directly with the taste cells.
- Taste buds are surrounded by basal cells (can differentiate into mature taste cells) and gustatory afferent axons which transmit gustatory information to other regions of the nervous system.
What three things contribute to our perception of flavour?
- smell
- touch (texture, temperature)
- taste
What type of receptor governs each type of taste receptor?
Ion Channel mechanisms:
- saltiness
- sourness
GPCR mechanisms (via T1 and T2 taste receptors):
- bitterness
- sweetness
- umami
Describe the taste transduction mechanism for saltiness.
The prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation sodium (Na+).
- Na+ passes through Na+ selective channels, down its concentration gradient into the cell
- This depolarises the taste cell, activating voltage-gated Ca2+ channels (VGCCs) (and voltage gated sodium channels to open)
- This triggers the release of neurotransmitters from synaptic vesicles which activate gustatory afferent axons that transmit information to other regions of the brain.
- Special Na+ selective channel (amiloride sensitive) are used to detect low concentrations of salt - these channels are insensitive to voltage and generally stay open.
Describe the taste transduction mechanism for sourness.
Protons (H+) ions are the determinants of acidity and sourness.
Pathway for sourness less well understood than for saltiness.
- Likely that H+ can pass through proton channels (from high concentration outside to low concentration inside) and bind to and block K+ selective channels
- This leads to depolarisation of the taste cell, activating VGSC (voltage gated sodium) and VGCCs causing them to open
- This triggers the release of neurotransmitters from the vesicles which activates gustatory afferent axons that transmit information to other regions of the brain.
Describe the taste transduction mechanism for bitterness.
T2Rs (GPCRs) are the receptors responsible for detecting bitter tastes. There are approximately 25 different T2Rs - since bitter receptors are considered ‘poison detectors’ it is important to have a large number of subtypes to detect a range of different poisonous substances.
- Bitter tastants binds to T2R, which is coupled to the G-protein Gq
- This G protein stimulates the enzyme phospholipase C (PLC), leading to the increase in production of the intracellular messenger inositol triphosphate (IP3)
- IP3 intracellularly activates a special type of Na+ ion channel (unique to taste cells), causing it to open and sodium to enter the cell. IP3 also triggers the release of Ca2+ from intracellular storage sites
- Both of these actions depolarise the taste cell - this triggers the release of ATP. ATP passes through ATP permeable channels and activates specific gustatory afferents axons which transmit taste information to other regions of the brain.
Describe the taste transduction mechanism for sweetness.
- Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is coupled to the G-protein Gq
- This G protein stimulates the enzyme phospholipase C (PLC), leading to the increase in production of the intracellular messenger inositol triphosphate (IP3)
- IP3 intracellularly activates a special type of Na+ ion channel (unique to taste cells), causing it to open and sodium to enter the cell. IP3 also triggers the release of Ca2+ from intracellular storage sites
- Both of these actions depolarise the taste cell - this triggers the release of ATP. ATP passes through ATP permeable channels and activates specific gustatory afferents axons which transmit taste information to other regions of the brain.
Describe the taste transduction mechanism for umami.
- Umami tastants bind to dimer receptor formed from T1R1 and T1R3, which is coupled to the G-protein Gq (umami shares a T1R3 protein with sweetness - this means T1R1 subunit determines specificity to umami as opposed to sweetness which has the T1R2 subunits)
- The G protein stimulates the enzyme phospholipase C (PLC), leading to the increase in production of the intracellular messenger inositol triphosphate (IP3)
- IP3 intracellularly activates a special type of Na+ ion channel (unique to taste cells), causing it to open and sodium to enter the cell. IP3 also triggers the release of Ca2+ from intracellular storage sites
- Both of these actions depolarise the taste cell - this triggers the release of ATP. ATP passes through ATP permeable channels and activates specific gustatory afferents axons which transmit taste information to other regions of the brain.
What is the flow of taste information to the CNS?
- Anterior tongue sends gustatory axons to a branch of cranial nerve VII (facial nerve).
- Posterior tongue sends gustatory axons to a branch of cranial nerve IX (glossopharyngeal nerve)
- Epiglottis sends gustatory axons to a branch of cranial nerve X (vagus nerve)
These synapse with the gustatory nucleus in the medulla where taste pathways then diverge through to the ventral posterior medial nucleus in the thalamus and then to the gustatory cortex which mediates the conscious experience of taste.
Is smell innate or learned?
- Smell preferences are inborn. In combination with taste, this helps to increase out enjoyment of foods.
- However like with taste, experience can strongly modify our innate preferences and olfaction can be trained e.g. the harmful smell of cigarette smoke for some people will encourage them to smoke, individuals that work in the perfume/whiskey industries are able to train their sense of smell to be finely tuned with different odours - they are able to distinguish between thousands of odours.
Describe the olfactory epithelium.
We smell with a small thin sheet high up in the nasal cavity called the olfactory epithelium. It has three main cell types - olfactory receptor cells, supporting cells and basal cells.
Olfactory Receptor Cells- site of smell transduction. They have axons that penetrate into the central nervous system
Supporting Cells - function to produce mucus - odorants (chemical stimulants) dissolve in mucus layer before reaching cilia of olfactory receptor cells. Mucus flows constantly and is replaced frequently. Mucus comprises a variety of proteins e.g. antibodies and enzymes which carry out important protective functions
Basal Cells - immature olfactory receptor cells that can differentiate into mature olfactory receptor cells. Olfactory receptor cells continuously grow, degenerate and regenerate (regularly replaced).
